1. Field of the Invention
This invention relates to an aqueous composition for forming an insulative glass coating directly on silicon steel strip and sheet stock, and more particularly to an aqueous magnesia slurry of high concentration wherein a substantial portion of the magnesia is an inactive magnesia (having a citric acid activity, as hereinafter defined, of greater than 200 seconds), and wherein a thermally decomposable phosphate containing compound is present as a stabilizing agent which keeps the inactive magnesia in suspension and increases the viscosity of the slurry. The invention further relates to a method of suspending an inactive magnesia in water to produce a stable slurry which can be readily applied to silicon steel surfaces to form a dried layer which will react with the steel to form a glass film during a subsequent high temperature anneal.
2. Description of the Prior Art
In the production of Oriented Silicon Steel decarburized strip and sheet stock is conventionally coated with an aqueous slurry of magnesia which is dried at low heat. During decarburization a layer of fayalite (an iron silicate) is formed on the silicon steel stock surfaces. The stock which is coated with a dried layer of magnesia is then subjected to a final, high temperature anneal at about 1095.degree. to about 1260.degree. C., during which the magnesia in the coating reacts with the fayalite layer to form a glass film, and the cube-on-edge orientation is developed by secondary recrystallization, as is well known in the art.
Heretofore, it has been considered necessary to use an active magnesia having a citric acid activity of less than 200 seconds, in order to provide an aqueous slurry from which the magnesia would not settle out rapidly and in order to obtain reaction between the magnesia and the fayalite surface layer. An active magnesia of the type conventionally used hydrates, with consequent increase in viscosity of the magnesia--water slurry, and this creates a problem since the viscosity of the slurry must be kept within a range which will permit application of a coating of uniform thickness by dipping, spraying, metering rolls, or the like.
Inactive magnesias have never previously been considered suitable for forming an insulative glass film on oriented silicon steel strip and sheet stock, because the dense particles could not be kept in suspension resulting in the formation of a slurry with very low viscosity. The dense inactive particles also would not react with the fayalite surface layer, at least within the time limits imposed by commercial production rates.
However, since inactive magnesias are available at much lower cost than the active magnesias, substitution of inactive magnesia in an annealing separator composition offers the prospect of very substantial economies in the processing of cube-on-edge oriented silicon steel.
Although additives to magnesia slurries have been proposed for the purpose of improving glass film properties and/or to facilitate the magnesia-fayalite reaction, to the best of applicant's knowledge no additives have been developed which are successful in stabilizing an aqueous slurry of an inactive magnesia and increasing the viscosity thereof.
The addition of phosphates to an active magnesia slurry is known, in order to improve the glass film properties and magnetic properties of the silicon steel base stock. Reference may be made to U.S. Pat. No. 3,615,918, issued to J. D. Evans and D. W. Taylor, wherein a magnesia composition containing a decomposable phosphate compound is disclosed. According to this patent, the phosphate, which ranges between 1% and 25% by weight calculated as P.sub.2 O.sub.5 is reduced to elemental phosphorus during the final high temperature anneal which diffuses inwardly from the coating into the silicon steel.
Phosphate base coatings which may be applied directly to metallic surfaces, or as secondary coatings over a mill glass magnesia-base coating, are known in the art. Phosphate and magnesia-containing coatings suitable for application to oriented silicon steel surfaces, either directly or as a secondary coating, are disclosed in U.S. Pat. Nos. 3,840,378 and 3,948,876, issued to J. D. Evans.
As indicated above, to the best of applicant's knowledge, the prior art has never suggested the use of inactive magnesia for annealing separator compositions on oriented silicon steel strip and sheet stock, nor have additives been proposed which would remedy the problems inherent in using an inactive magnesia for this purpose.